1. Field of the Invention
The present invention relates to a dual access control system for a magnetic-disk unit, particularly to a dual channel magnetic-disk unit (hereinafter referred to as a magnetic-disk unit) accessible by two magnetic-disk control units (hereinafter referred to as control units).
2. Description of the Related Art
A dual access control system of this type has been described in a document entitled "CDC STORAGE MODULE DRIVE Pub. No. 83322320" published in 1983 by Control Data Corporation.
The dual access control system will be described with reference to this document in the case where two control units compete with each other.
A process for controlling one magnetic-disk unit by two magnetic-disk control units comprises a reserve function for reserving the magnetic disk (hereinafter referred to as reserve), a release function for releasing the magnetic-disk unit from a reserved condition (hereinafter referred to as release), and a force function for causing the magnetic-disk unit to be reserved (hereinafter referred to as priority select). Execution of the reserve, the release and the priority select functions is controlled by a combination of signals, i.e., a unit select signal, a bus bit signal, and a tag signal, sent between the magnetic-disk unit and the control units.
FIG. 14 is a system arrangement employing a dual access control system for a prior art magnetic-disk unit.
A central processing unit (hereinafter referred to as CPU) 64, a main memory (hereinafter referred to as MEM) 65, and one control unit 68 are connected with each other on a line 74 via lines 71 to 73 while a CPU 66, an MEM 67, and another control unit 69 are connected with each other on a line 80 via lines 77 to 79. The control units 68, 69 are connected with a magnetic-disk unit 70 by lines 75, 76.
FIG. 15 is a prior art block diagram illustrating the flow of interface signals in the dual access control system of FIG. 14.
A host unit 62 comprising the CPU 64 and the MEM 65 is connected via line 73 to the control unit 68 comprising a processing unit 81 for controlling the magnetic-disk unit 70, a control memory 82 for storing microprogram comprising a magnetic disk control procedure, and a magnetic-disk control part 83. Interface signal lines 75 are positioned between the control unit 68 and the magnetic-disk unit 70 and include interface control signals such as a unit select tag signal (hereinafter referred to as the USTG signal) for connecting the magnetic disk control part 83 with the magnetic-disk unit 70.
The operation of the dual access control system having such structure will now be described.
If a reserve, release or priority select command is sent from the CPU 64 to the control unit 68, the control unit 68 supplies control signals to the magnetic-disk unit 70 by way of the interface lines 75. The control signals include the USTG signal, which is a magnetic disk select signal and constitutes a tag signal of a unit select 2.sup.0 to 2.sup.2 signal (hereinafter referred to as UNIT SELECT 2.sup.0 to 2.sup.2 signal) The UNIT SELECT 2.sup.0 to 2.sup.2 signal designates a magnetic disk number and is significant (i.e., contains valid data) only when the USTG signal is enabled. A tag 3 signal (hereinafter referred to as the TAG 3 signal) is a tag signal of a bus bit 9 signal (hereinafter referred to as BUS BIT 9 signal) for causing release of the magnetic-disk unit when the TAG 3 signal is effective (i.e., enabled). Both a unit selected signal (hereinafter referred to as the SELECTED signal) and a busy signal (hereinafter referred to as the BUSY signal) are response signals of the magnetic-disk unit 70 for responding to the USTG signal.
FIG. 16 is a prior art timing diagram illustrating a reserve sequence and priority select sequence of the dual access control system of FIG. 14.
When the UNIT SELECTED signal goes "1", the magnetic-disk unit is reserved. When the BUSY signal goes "1", the magnetic-disk unit is not reserved since the same disk is reserved by the other control unit. As far as the priority select command is concerned, the magnetic-disk unit is reserved even if the same magnetic-disk unit is reserved by another control unit.
FIG. 17 is a timing diagram illustrating a release sequence of the dual access control system of FIG. 14.
Referring to the timing diagram, if the TAG 3 signal and the BUS BIT 9 signal are issued relative to the release command, the reserved magnetic-disk unit is released.
Inasmuch as hard disk controllers available in the present market are not provided with the functions of reserve, release, and priority select, the control units receive the commands involved in selecting the reserve, release, priority select functions and execute the functions on the basis of a microprogram control system provided with a control memory and a control processor respectively incorporated in the control units.
Prior art dual access control systems for magnetic-disk units typically require firmware control circuits. In contrast, microcomputer systems are characterized in that control functions are performed in software (as much as possible) in order to minimize the amount and cost of hardware. Providing the above-mentioned firmware control circuits in microcomputer systems would so increase the size and cost of hardware (i.e., the scale of hardware) that the advantageous features of the microcomputer systems would be lost. Thus, a problem exists because conventional dual access control systems (typically implemented using firmware control circuits) cannot be easily or advantageously used in microcomputer systems.